A conventional oscillation amplifying circuit using a MOSFET, which is incorporated in a semiconductor integrated circuit device, uses a circuit based on a CMOS inverter circuit typified by the circuit construction as shown in FIGS. 2 and 3. An amplifying circuit 50 shown in FIGS. 2 and 3 includes a P-type MOSFET 51 and an N-type MOSFET 52. The gate electrodes of these MOSFETs are connected to each other and the drain electrodes are also connected to each other. The circuit shown in FIG. 3 further includes a switching circuit 60 having a single P-type MOSFET 61. In the case where the amplifying circuit 50 is operated at a low voltage, each of the MOSFETs 51 and 52 is channel-doped so as to specially lower the threshold voltage.
Further, the miniaturization of the integrated circuit has been progressed in recent years, and it is required to lower the power source voltage in order to prevent the elevation in the intensity of the electric field involved in the development of the miniaturization. Because of the increase in demand for portable electronic instruments, an integrated circuit operable at a low voltage is required to achieve the power saving. In general, it is desirable to decrease the threshold voltage of the MOSFET in order to make the integrated circuit operable at a low voltage. A simple logic circuit can be operated if the power source voltage exceeds the threshold voltage of the P-type MOSFET or the N-type MOSFET, whichever of a higher threshold voltage. However, an oscillation circuit using a CMOS inverter circuit as an amplifying circuit is not operated if the power source voltage is not higher than the sum of the threshold voltages of the P-type MOSFET and the N-type MOSFET, due to the characteristics as an analog circuit. Therefore, for the low voltage oscillation, it is necessary to make the threshold voltage lower than the case of a logic circuit. However, if the threshold voltage is made very low, the voltage-current characteristics of the MOSFET can not necessarily permit the MOSFET to satisfactorily perform a sharp ON/OFF position against the gate voltage in general, even if the threshold voltage is set at a positive value. It follows that a leakage current takes place to some extent during OFF time of the MOSFET.
Under the circumstances, various problems are brought about if the threshold voltage is lowered in the conventional circuit construction described above in order to permit the circuit to be operated at a low voltage. The problems are such that, for example, the leakage current is increased during the OFF time, the yield in the manufacturing process is lowered, the channel doping step has to be added twice, leading to a high manufacturing cost, or a clock signal has to be supplied to the oscillating circuit that is separated from other components.
The present invention is devised to overcome these problems and has an objective thereof the provision of a low voltage oscillation amplifying circuit and a portable electronic instrument using the circuit. This low voltage oscillation amplifying circuit can be operated at a low voltage, can prevent leakage current when an operation of the circuit is stopped, and does not incur an increased manufacturing cost derived from an additional channel-doping step.